Priority is claimed to Patent Application Number 2001-1352, filed in the Republic of Korea on Jan. 10, 2001, herein incorporated by reference.
1. Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly, to a perpendicular magnetic recording medium which is used in hard disk drives (HDDs) and is capable of increasing thermal stability and a signal to noise ratio (SNR) when recording information.
2. Description of the Related Art
Hard disk drives (HDDs), the main apparatuses for storing information have been continuously evolving to meet ever-increasing demands for high density and lower price. Recently areal recording density has been increasing by more than 100% annually due to the developments of giant magnetoresistive (GMR) read heads, improved recording media and improved signal processing methods such as partial response maximum likelihood (PRML). For high density information recording, magnetic disks for HDDs must have low noise characteristics and at the same time must have good thermal stability to overcome a superparamagnetic effect.
FIG. 1 is a schematic sectional view of a conventional perpendicular magnetic recording medium. A template (see FIG. 1) underlayer 12 to induce perpendicular orientation of a perpendicular magnetic recording layer, perpendicular magnetic recording layer 13, a protective layer 14 to protect the perpendicular magnetic recording layer 13 from oxidation and mechanical wear, and a lubricant layer 15 are stacked in that order on a glass or aluminum (Al) alloy substrate 10.
A process for manufacturing a perpendicular magnetic recording medium employing a conventional Co alloy recording layer will be described below with reference to FIG. 1.
CoCr or CoPt based ternary or quaternary alloys are used to form the perpendicular magnetic recording layer 13, and the perpendicular magnetic recording layer 13 must be formed such that a [0001] axis of Co hexagonal grains are oriented perpendicular to the surface of the substrate 10. For this purpose, an underlayer 12 formed of Ti or Ti alloy such as TiCr is used as a template for oriented growth of the grains of CoCrPt magnetic layers.
In general, the substrate 10 may be a glass disk, a NiP coated Alxe2x80x94Mg disk or a thermally oxidized silicon disk. The Ti underlayer 12 is formed by depositing Ti or TiCr on the substrate 10 by sputtering or another physical deposition method. The thickness of the underlayer is in the range of 1-200 nm and the Co alloy perpendicular magnetic recording layer 13 is formed on the underlayer 12. Here, it is important that the [0001] side of the Ti crystal grains are oriented perpendicular to the substrate surface.
In perpendicular recording, a thicker magnetic layer used in longitudinal recording media can be used for higher density recording, which is a big advantage from a thermal stability point of view. However, it is well known that a medium noise level increases with increased recording density in a conventional perpendicular recording medium, more so than in a longitudinal recording medium. In the case of longitudinal recording, transition noise, noise occurring in the region where magnetic polarity of a recorded bit changes is an important medium noise. In perpendicular recording, direct current (DC)-erased noise, noise caused by reversed domains in a recorded bit as well as transition noise become important noise sources. In order to reduce the DC-erased noise, the number of the reversed domains in the recorded bit must be reduced.
The reduction of DC-erased noise can be achieved only if a xe2x80x9cnucleation fieldxe2x80x9d of a medium exists in the second quadrant of a major hysteresis loop. The nucleation field is an external magnetic field which must be applied to initiate reversal of magnetization after saturation in one direction (see FIG. 2).
In FIG. 2, a vertical axis M is magnetization, and the horizontal axis H is applied external magnetic field. The conventional perpendicular magnetic recording media of CoCr, CoCrPt and CoCrPtX types, the most studied ones, were designed to be much thicker than a longitudinal recording medium, for higher thermal stability. The thickness explored by most investigators is in the 50 to 200 nm magnetic thickness range. A squareness ratio (SQ) of the conventional medium has a value of 0.4 to 0.8. The SQ is defined as:       Squareness    ⁢          xe2x80x83        ⁢    ratio    ⁢          xe2x80x83        ⁢          (      SQ      )        =            remanent      ⁢              xe2x80x83            ⁢              magnetization        ⁡                  (                      M            r                    )                            saturation      ⁢              xe2x80x83            ⁢              magnetization        ⁡                  (                      M            s                    )                    
The use of a thick magnetic recording layer in perpendicular recording is based on the idea of a higher output signal and good thermal stability. However, if the thickness of the CoCrPt, CoCr, CoCrPtX alloys is greater, say beyond 50 nm, although the critical thickness depends on the composition of the magnetic layer and deposition conditions, the magnetization reversal mechanism changes from Stoner-Wolfarth type coherent rotation to incoherent rotation (Ref 1: Taek Dong Lee, Min Sig Hwang, Kyung Jin Lee, xe2x80x9cEffects of magnetic layer thickness on negative nucleation field and Cr segregation behavior in CoCrPt/Ti perpendicular mediaxe2x80x9d, Journal of Magnetism and Magnetic Materials, vol. 235(2001), p. 297-304; K. J. Lee, T. D. Lee, N.Y. Park, xe2x80x9cstep-like energy barrier variation of high Ku materialsxe2x80x9d, Digest submitted abstract for Intermag 2002). This reduces the magnitude of a negative nucleation field.
When the squareness ratio is less than 1, the magnetization reversal is initiated under a positive applied field due to a self-demagnetization field created during reduction of the applied field after saturation in a positive applied field in perpendicular recording media.
However, when the squareness ratio is 1, the magnetization reversal is initiated under zero or negative applied field during reduction of the applied field after saturation in a positive applied field. Thus, when squareness is less than 1, the nucleation field for magnetic reversal is a positive applied field and we define this as xe2x80x9cpositive nucleation fieldxe2x80x9d. If the squareness is 1, the nucleation field for magnetic reversal is zero or a negative applied field and we define this as a xe2x80x9cnegative nucleation fieldxe2x80x9d.
In a medium with a positive nucleation field, magnetization reversal will occur even in a state where an external applied field is not applied, and this serves as the source of DC-erased noise. In media with 0 nucleation field or a very small negative nucleation field, magnetization reversal within a written bit also occurs due to a demagnetization field as shown in Ref. 2 (L. Wu, N. Honda, K. Ouchi, xe2x80x9cLow noise Co/Pd multiplayer media for perpendicular magnetic recordingxe2x80x9d, IEEE Trans. Magn., vol. 35(1999), p. 2775-2777).
Therefore, to reduce the DC-erased noise, a medium with a substantial negative nucleation field is necessary (Ref. 2).
In addition to this, when a ring head is used with a single layer perpendicular medium without a proper negative nucleation field, magnetization of small grains in a penultimate bit can be reversed by a reversed head field during writing a new bit as the head field has a wide distribution.
The latter phenomenon occurs more significantly in high density recording and thus, the conventional perpendicular recording media with 0 or a positive nucleation field cannot be used in a high density recording region.
To solve the above problems in the conventional perpendicular media, it is an object of the present invention to provide a CoCrPt-type perpendicular magnetic recording medium with a negative nucleation field stronger than xe2x88x92500 Oe. This will provide high thermal stability in a low density recording region and low noise properties in a high density recording region.
Accordingly, to achieve the above object, there is provided a perpendicular magnetic recording medium in which an underlayer for leading perpendicular orientation of a perpendicular magnetic recording layer is stacked between a substrate and the perpendicular magnetic recording layer, and the thickness of the perpendicular magnetic recording layer is controlled within the range of 5-40 nm to have a negative nucleation field.
It is preferable that the perpendicular magnetic recording layer is formed of CoCrPt alloys.
It is prefarable that the perpendicular magnetic recording layer contains 8-20 at % Pt depending on the content of Cr in the layer.
It is also preferable that the perpendicular magnetic recording layer contains 12-20 at % Cr.
It is also preferable that the perpendicular magnetic recording layer further comprises Ta, Nb or Ta+Nb in an amount of less than 4 at %.
It is also preferable that the under layer is formed of Ti-alloy.